Mihai Dimonie

Inverse suspension polymerization of acrylamide

Abstract The polymerization of acrylamide in inverse suspension is described and compared with polymerization in solution. The influences of various factors upon the molecular weight of the polymer were studied, viz. the concentration of initiator, the procedure of aqueous phase addition, the nature and concentration of emulsifier, the salt concentration, the time between mixing of phases and start of polymerization etc. Conductometry, NMR and electron microscopy were employed in order to establish the reaction stages and the characteristics of the polymers. The results indicate the importance of each factor and lead to an understanding of the polymerization.

New catalysts for linear polydicyclopentadiene synthesis

Abstract The ring-opening polymerization of dicyclopentadiene (DCPD) may occur in two ways: in the first way the reaction occurs by opening of the norbornene ring, in the second one both rings, norbornene and cyclopentene are involved. The polymerization of DCPD via opening of norbornene moiety implies the utilization of very selective catalytic systems. In our work, several new catalytic systems for synthesis of linear polydicyclopentadiene, derived from WCl 6 and WOCl 4 and organosilicon compounds have been employed. The microstructure of the linear polydicyclopentadiene (prepared from endo -DCPD) was evaluated from IR, and 13 C -NMR spectroscopy, showing that the polymer has prevailingly “ cis ” double bond configuration. The thermal behavior has been followed by differential scanning calorimetry. The linear polydicyclopentadiene obtained under these conditions has a glass-transition temperature of 53°C.

Relationship between structure and stereoselectivity in ring-opening metathesis polymerization catalytic systems

Abstract Polypentenylenes of varying cis and trans steric configurations have been synthesized from cyclopentene using binary and ternary catalytic systems based on WCl6 and three classes of organometallic compounds: organoaluminium, organotin and organosilicon. In several cases the distribution of the double-bond pair sequences has been determined from the fine structure of the 13C NMR spectra. Analysis of the polymer microstructure indicated that the catalytic systems derived from WCl6 and organoaluminium compounds (Bui3Al, Et3Al, ET2AlCl and EtAlCl2) gave mainly trans configuration of the carbon-carbon double bond and a random or slightly blocky distribution of the double-bond dyads. By contrast, catalysts derived from WCl6 and organotin (Et4Sn, Bu4Sn and Ph4Sn) and organosilicon (Me2(allyl)2Si) compounds led primarily to cis configuration of the carbon-carbon double bond and a blocky or slightly blocky distribution of the double-bond dyads. In addition, the catalysts based on WCl6 and aluminoxane (diisobutylaluminoxane) exhibited a rather high cis stereospecificity in cyclopentene polymerization. ESR and UV-Vis investigations as well as ceriometric determinations performed on WCl6/Bui3Al, WCl6/Bui2AlOAlBui2, WCl6/Me2(allyl)2Si and WCl6/Ph4Sn afforded new structural data concerning the oxidation states of the W atom and the interaction between the catalytic components. These results indicated that (if other parameters, e.g., temperature, time, activators, solvent, are not considered) there exists a direct correlation between the oxidation state of the W species generated in these systems and the polymer microstructure. These data allowed proposals to be advanced concerning the structure of the active species, the mechanism and the stereochemistry of the ring-opening polymerization induced by the above catalysts.

New aspects of the mechanism of the ring-opening polymerization of cyclo-olefins

Abstract The polymerization activity of the cyclopentene/WCl6 system initiated by water has been correlated with the occurrence of paramagnetic species involving the lower oxidation states of tungsten as detected by ESR spectroscopy. The W(III) species formed by the cyclo-olefin-controlled reduction of WCl6 were shown to be the precursors of the active species in polymerization reactions initiated by water in the absence of the alkylaluminium compound as a carbene source. The same W(III) species are assumed to be the precursor of the active species in the cyclopentene polymerization initiated by the WCl6/i-Bu3Al system. Attempts to polymerize cyclohexene in the presence of WCl6/i-Bu3Al failed; however, ESR signals assigned to W(V) and W(III) species were recorded. Independent studies carried out by mass spectrometry for the products resulting from the interaction between cyclopentene or cyclohexene and WCl6 demonstrated the existence of well-defined cyclic hydrocarbons along with 3-chlorocyclopentene and 3-chlorocyclohexene, respectively. The formation of these compounds may be explained via a cationic mechanism. The mechanisms of the initiation and propagation steps in polymerizations generated by carbene species from cyclo-olefin and WCl6 in the absence of alkylaluminium compounds are discussed. The role of the cyclo-olefin for reducing the tungsten compounds and producing the metallacarbene species is outlined. The sharp influence of strongly complexing additives on the reaction kinetics, catalyst activity and on the evolution of molecular weight with time in the polymerization of cyclopentene is presented.

ESR investigation of the interaction between WCl6-based catalysts and cyclo-olefins

Abstract Tungsten hexachloride, after 20 min contact with cyclopentene at room temperature, exhibited an ESR signal with a g-factor of 1.918 ± 0.004, aw = 70 G, ΔH

Stereoselectivity of cycloolefin polymerization with WCl6-based catalytic systems

25 G, assigned to a W(V) species. The intensity of this signal increased with time and showed a maximum after 200 min. After 72 h interaction, a second signal with a g-factor of 1.680 ± 0.04, ΔH = 110 ± 10 G, assigned to a W(III) species was detected. In a control experiment, the same W(III) signal was recorded for a reduced tungsten compound (in aqueous medium) whose oxidation state was determined by cerium sulphate titration. The above W(V) signal also resulted from the interaction of WCI6 with cyclohexene, cyclo-octene and cyclododecene. The variation with time of the intensity of this signal was characteristic for each cyclo-olefin. Higher reduction rates were observed for cyclopentene and cyclo-octene. Three distinct ESR signals were recorded during the polymerization of cyclopentene in the presence of the WCl6/epichlorohydrin/i-Bu3Al system. The first signal (g= 1.955 ± 0.005, ΔH =65 G) was assigned to a W(V) species, the second (g = 1.720 ± 0.03, ΔH = 45 G) to a W(III) species and the third (g = 2.018 ± 0.002, ΔH = 30 G) to a hydrocarbon radical species. The intensity of the W(III) species was related to the polymerization activity while that of the hydrocarbon radical species was related to catalyst deactivation. The ESR spectrum obtained during the interaction of cyclohexene with the WCl6/epichlorohydrin/i-Bu3Al system exhibited three characteristic signals (g1 = 2.008 ± 0.001, ΔH1 = 12 G;g2 = 1.830 ± O.02, ΔH2 = 60-100 G; g3 = 1.740 ± 0.02, ΔH3 = 40 G) assigned to the W(V) and W(III) species.

Ring-opening polymerization of cycloolefin induced by tungsten porphyrinates

Abstract The stereochemistry of ring-opening metathesis polymerization of several cycloolefins (cyclopentene, cyclooctene, 1,5-cyclooctadiene, cyclododecene) in the presence of WCl6-based catalysts (WCl6/iBu3Al/epichlorohydrin, WCl6/iBu3Al/chloranil, WCl6/phenylacetylene, WCl6/Ph4Sn, WCl6/allyl4Si, WCl6/iBu2AlOAliBu2) has been carefully examined. The product stereoselectivity and polymer microstructure were greatly dependent on the nature of the catalytic system and reaction conditions; the nature of the cycloolefin also influenced the steric course of the reaction. An important stereochemical consequence was that the cis and trans content of the polyalkenamer greatly depended on the oxidation state of the W-atom, catalyst activity and reaction temperature. The relationship between the microstructure of several polyalkenamers (polypentenamer, polyoctenamer, polydodecenamer) obtained with the above catalytic systems and cycloolefin reactivity or catalyst nature is fully illustrated. The role of the ligand as well as of the cocatalyst and activator is also shown. The particular stereochemical behaviour of cycloolefins in the presence of the studied WCl6-based catalytic systems is discussed in terms of the metallacarbene—metallacyclobutane mechanism. The steric effects induced by the coordinating cycloolefin and the existing ligands at the transition metal are also considered.

Influence of WCl6-based catalytic systems on polypentenamer microstructure

Abstract Polymerization reactions of cyclopentene, cyclooctene and cyclododecene have been carried out in presence of catalytic systems based on tungsten tetraphenylporphyrinate, under inert atmosphere at room temperature, in aromatic solvents. The catalyst precursor was prepared by interaction of WCl 6 with free tetraphenylporphyrin in CCl 4 and characterized by spectroscopic methods (e.g. UV—Vis etc.). Spectroscopic studies on the interaction between the catalytic components or with the monomer, as well as kinetic results, molecular weight distribution and polymer microstructure indicated a high stability and efficiency of the catalytic systems employed. This catalyst allowed polyalkenamers with monomodal and narrow molecular weight distribution to be obtained. The polypentenamer displayed a block and/or alternate distribution of the carbon—carbon double bond dyads and high trans configuration. Data obtained under the above conditions gave information concerning the nature of active species during the initiation and propagation reactions.

On the oxidation state of the tungsten atom in WCl6-based metathesis catalysts used for the ring-opening polymerization of cycloolefins

Abstract The microstructure of polypentenamer resulting from cyclopentene polymerization initiated by two types of WCl 6 -based catalytic systems has been determined from the 13 C NMR spectrum. Ternary catalysts containing organoaluminium compounds as cocatalysts ( e.g. WCl 6 -epichlorohydrini-Bu 3 Al) lead mostly to polypentenamers of predominant trans configuration. However the nature of the activator affects the polymer microstructure; this microstructure was correlated with the reducing ability of the organoaluminium compound and the electron donor, and the acceptor ability of the activator. In contrast, the binary catalyst containing organotin as cocatalyst (WCl 6 -Ph 4 Sn) yields essentially polypentenamer of mainly cis configuration at −20 °C, the cis content falling at higher temperature. Analysis of double-bond pair sequences in the polymer obtained with the above catalysts showed a non-Bemoullian distribution for cis contents greater than 30%. Determination of the oxidation states of W in the active species by ceriometric titrations indicated different valency states for the two types of catalyst; the different microstructures of the polymer were correlated with these distinct valencies of the W atom. The above results are interpreted in terms of a metallacarbene-metallacy do butane mechanism; the main stereochemical features of the ringopening polymerization of cyclopentene using the two types of catalyst are outlined. The essential role of the oxidation state of the W atom for reaction stereochemistry is pointed out.

Specific behaviour of cycloolefins in ring-opening polymerisation

The oxidation state of the tungsten atom in the active species of homogeneous WCl6 -based catalytic systems used for the ring-opening polymerisation of cycloolefins (such as cyclopentene, cyclooctene, 1,5-cyclooctadiene and cyclododecene) was found to vary from W(III) to W(VI), depending essentially upon the nature of the organometallic compound employed as the cocatalyst. The oxidation state has been established from ceriometric determinations and parallel ESR measurements. The data obtained indicated that the catalysts derived from WCl6 and organoaluminium compounds (R3Al or R3−xAlClx where R = Me, Et or i-Bu) contained mainly W(III) as the active species. Those derived from WCl6 and organotin compounds (R4Sn where R = Me, Et or Ph) contained W(IV), W(V) and/or W(VI), whereas the catalysts obtained from WCl<6 and organosilicon compounds exhibited mainly W(VI) as the active species. In addition, it was found that the oxidation state of the transition metal strictly governed the sterochemistry of the reaction. The essential role of the organometallic compound, as well as that of the unsaturated substrate, in generating various oxidation states of the tungsten atom in the catalytic systems is outlined, together with the relationship between the particular oxidation state of the transition metal, the catalyst activity and the reaction stereochemistry. The results are interpreted in terms of the metallacarbene mechanism involving different geometric configurations of the active species which correspond to the relevant oxidation states of the tungsten atom.